1. Field of the Invention
The present invention relates to a process and an apparatus for controlling spark erosion processes, i.e. of cutting and vertical erosion processes.
Spark erosion processes are used for machining of electrically conductive workpieces with high precision by means of electrodes. The resulting shape of the workpiece is determined by various factors, like the shape of the electrodes, the machining force, etc. A preferably constant and stable response of the working electrode as well as an accurate position and movement control are required for obtaining a high quality during machining, specifically for setting the proper precision; it is particularly important to maintain the distance between the electrode and the section of the workpiece which is to be machined, as accurate as possible, subject to the desired process conditions; and also the corresponding machining time and machining intensity. This causes problems particularly with extremely thin and flexible electrodes of the kind which are used in the field of microerosion, for instance for microdrilling. Problems arise especially in the area of handling of malfunctions, like short circuits.
2. Description of the Related Art
CH-PS-525 061 (KONDO) describes in general the basis for the automatic control of spark erosion processes. These comprise, in particular, sensing means for measuring voltages and currents, a logic circuit for processing the measured information and control means, hereby controlling the spark erosion processes. The movement of the electrode occurs in relation to the actual conductivity of the gap (i.e. the conductivity in the dielectric between the electrode and the workpiece) by means of a servo mechanism. Malfunctions, like short circuits or arcing, are corrected, among others, by limiting the short circuit current and by moving the electrode away from the workpiece.
EP-A-0 333 170 (AGIE) and U.S. Pat. No. 4,864,091 compare a desired distance which corresponds to the present processing conditions for machining the workpiece, with the actual distance between the working electrode and the workpiece. An error signal is computed by comparing these two values, and this error signal is transmitted by means of a low-pass filter circuit to a control circuit for moving the electrode. The actual distance is preferably determined from the present erosion voltage.
U.S. Pat. No. 5,159,167 (CHAIKIN et al.) refers to, among other things, the control of the position of an electrode by means of spark sensors and to a precise control of the hole size by means of a "complete" control of the electrical discharge.
DE 32 04 799 A1 (YAMADA et al.) refers to an apparatus for machining workpieces by electrical discharge, where the apparatus switches off automatically at the "end of a machining process", i.e., exactly at the moment when the electrode has penetrated the workpiece.
DE 33 00 552 C2 (GARBAJS et al.) teaches, in principle, a method for controlling a process for machining by means of spark erosion, whereby the discharge pulses which are present in the gap between a tool and the workpiece to be machined, are detected and their characteristic voltage profile is analyzed. Furthermore, the distinction is made between abnormal discharges and short circuit pulses due to galvanic short circuits, causing the machine to respond differently.
In many situations, these known apparatuses and/or methods are unable to react adequately to sudden changes in the erosion conditions, for instance to the sudden changes which occur when a microdrilling electrode passes through the underside of the workpiece. When this happens, the flushing fluid for the erosion can escape through the underside, or, in more general terms, can flow through the workpiece. This causes a drastic change in the flushing pressure, the flushing speed and the direction of motion of the flushing fluid. Consequently, the position of the electrode will be unstable, gas bubbles will form in the drilled cavity, and the particles which were already eroded, if they are still within the drilled cavity, will change their direction of motion and will be flushed downwards. This will lead to "non-mechanical" and "other" short circuits, abrupt variations in the voltage and errors in the measured control values, since the voltage across the gap looses its customary meaning (proportional to the gap width). The known control systems reacts in this situation by rapidly retracting the electrode, whereby the instability in the machining process is enhanced or the process terminated, regardless if the actual results correspond to the desired results or not. The first situation in particular will lead to an increased residence time of the electrode in the exit area of the drilled hole. Since the known methods and/or apparatuses are not able to take this extreme situation as such into account, erosion pulses will continue to be emitted during the instability phase, whereby the hole diameter increases in an irregular, undesirable, and uncontrollable fashion.